It is a known technique to remove flash from molded articles and paint or other coatings from various articles by contact with a chilling medium at low temperature effecting the embrittlement of the flash or coating, thereby facilitating removal of the flash or coating by impact.
In certain of such systems the impact is effected by tumbling the embrittled articles in a rotating drum. Such a system is illustrated in U.S. Pat. No. 3,468,077, as applied to deflashing of molded articles.
The removal of layers of surface coatings of organic material built up on a support, by embrittlement of the coating to lessen the bonded relation between the support and the built up layers, is disclosed in U.S. Pat. No. 3,934,379. The patent indicates that the embrittled coating may be separated from the support by abrasion or impact. No particular form of apparatus is illustrated in the patent, but the embrittlement is described as being carried out by full or partial immersion of the coated article in a bath of liquefied gas, or by spraying the liquefied gas directly on the article. Removal of the embrittled coating can then be effected by means of a blast of abrasive material directed thereon by a conventional air gun or by using a known type of centrifugal wheel hurling abrasive particles, such as sand or metal shot, radially outwardly at high velocity. For thick layers of coating the patent advocates impact of the embrittled coating by striking with a hammer or the like.
Low temperature processes and systems for deflashing of molded resilient articles by physical impact of the embrittled flash are known in the prior patent art. In U.S. Pat. No. 3,468,077, removal of the selectively embrittled flash is preferably effected by drum tumbling, with the suggestion that the temperature control instrumentation therein disclosed may be used to control other types of mechanical deflashing means such as shot peening and vibrating equipment. Specific systems for flash removal from cryogenically precooled articles by shot blasting are disclosed more particularly in U.S. Pat. Nos. 4,312,156 and 4,355,488. A preferred blasting medium is pelleted polycarbonate resin, as disclosed in U.S. Pat. No. 3,313,067.
In typical operation of such systems the articles to be treated are introduced or placed into a thermally insulated chamber maintained at the required low temperature effecting the desired embrittlement of the portions to be removed and the stream of blasting media is centrifugally impelled at high velocity against these articles by one or more rotating impellers or so-called throwing wheels. The discharged blasting media together with the fragments of flash or coating material removed thereby, are collected and conveyed out of the treating chamber to a screening apparatus in which the blasting media is separated and recovered for recycling to the blasting operation, while the larger fragments of the removed flash or coatings as well as the fines are discharged.
While there are certain features had in common in systems for removal of coatings as well as in systems for deflashing, each of these operations presents its individual problems, as will appear below.
Improvements in systems designed more particularly for coating removal by embrittlement and blasting with impact media are disclosed, for example, in pending patent applications. Ser. No. 445,778 filed Nov. 30, 1982 and in Ser. No. 461,087 filed Jan. 26, 1983.
In the case of deflashing of molded articles the composition of the flash is not different from that of the body of the article, so that the selective embrittlement of the flash depends upon the relative thinness of the flash portion. In the case of coated articles the tenaciously adhering layers of the coating bear no significant relation to the composition of the supporting base to which these are bonded. The thermal contraction of organic coatings is much greater than that of metallic structures bearing such coatings when they are both cooled to the same temperature. However, when a coated article is cooled very rapidly, the coating will be substantially colder than the article itself. Thus, to maximize the differential thermal contraction, it is necessary to achieve the fastest possible cooling rate of the coated articles. This can be accomplished by subjecting the article to the lowest possible temperature and by increasing the heat transfer coefficient on the surface of the coated article.
After the bonding interface between the coating and the article has been weakened by the shear stress produced by differential thermal contraction, the coating is removed by impacting the surface with high velocity media. When the coating is breaking away from the article during this phase of the process, the coating can be removed more easily if the material is at or below its embrittlement temperature. However, for most coating materials the embrittlement temperature is significantly warmer than the temperature utilized during the very rapid cooling phase. Thus, to minimize the consumption of refrigerant, such as liquid nitrogen (LIN), it is desirable to operate the media blasting phase of the cycle at a warmer temperature. Finally, when the coating removal phase is partially completed, further reduction in consumption of refrigerant can be had by shutting off the input of LIN.
The conventional process control for cryogenic coating removal has several disadvantages that limit the productivity of the system and lead to increased consumption of refrigerant. In these conventional systems an automatic cycle is utilized, wherein a timed prechill period is initiated by the operator by energizing the system. During this period the cryogenic liquid refrigerant is injected into the treating chamber to lower the chamber temperature to a preset value, maintained by a temperature controller. When the set prechill period is completed, a media blast timer and a cycle temperture timer are energized. During the time period preset for the media blast, the throwing wheels and the media feeders are operated to impact the coated articles with high velocity particles, during which period the temperature controller continues to maintain the preset temperature level in the chamber. When the set cycle temperature time period is completed, the supply of LIN to the treating chamber is discontinued but the blasting of the coated article with media is continued for the additional preset time period. At the completion of such additional blasting period, the automatic cycle is completed, the system stops its operations, and the chamber may be unloaded. These conventional cryogenic coating removal systems employ a fixed time period for prechilling the workpieces and a fixed period in the operating cycle during impacting, both operated at a single constant temperature environment.
In these conventional systems, the time required to lower the chamber temperature to the preset value will vary for a number of reasons. The cooldown of the treating chamber, the chamber insulation, the media, the media separator and the media feeders, is a profressive process requiring six to nine cycles to reach a constant time period. Thus, the initial cycles will experience a warmer temperature-time profile resulting in incomplete coating removal. When the system reaches the constant cooldown time period after many cycles, the automatic cycle will be longer than necessary, thus wasting refrigerant and production time. During the progressive system cooldown, the operator should continually readjust the prechill time period. The cooldown time will also be affected by the saturation condition of the LIN or other refrigerant in the storage tank from which it is supplied to the treating chamber. Thus, for example, as the LIN storage tank pressure increases, the available refrigeration capacity of the LIN will decrease, causing the cooldown time to vary. Further, as the size of the workload increases, the cooldown time will also increase. The many variations in the temperature-time profile encountered by the coated articles will cause erratic and inconsistent decoating results.
Many of the disadvantages encountered in the conventional control applied to the design and operation of cryogenic decoating systems, stems from the fact that such control is based on process control arrangements utilized for the deflashing of molded articles. The deflashing process requires cooling of the thin flash to just below the embrittlement temperature and impacting the brittle flash with high velocity media. However, if the molded article is subjected to a substantially colder temperature, cracking and product damage will result. Since the cooling rate will be short of maximum when the treating chamber is just below the temperature of embrittlement, the decoating process is retarded and the coating must be mechanically abraded by the media blast. The longer cycle time thus required will decrease the productivity of the system and increase consumption of refrigerant.
Among the objectives of the present invention is the provision of a process control system wherein the cryogenic removal of coatings by embrittlement and impact can be operated at improved efficiency, overcoming certain of the disadvantages experienced in conventional prior art systems and obtaining significantly higher productivity while reducing consumption of refrigerant.